The microwave heating or baking of various products is performed by placing the products on a support which is in turn placed in a closed application cavity.
Although it is possible, by applying microwaves, to heat or to bake a product, it has however been found that in proceeding in this way, the heated or baked product, no longer has the same qualities as those exhibited by the same product which has been heated or baked by different means.
This is especially so in the preferred hut non-exclusive field of application which is that of heating and baking of food products, known to lose their organoleptic and/or nutritional qualities when they have been subjected to the action of microwaves.
The physical nature of certain food products can even, in some cases, be affected by splitting or bursting of the outside skins, teguments, pods or other shells.
This seems to be due to the disturbance of the free and trapped molecules of water whose dissociation causes the release of diffusing vapors, carrying with them some at least of the principles of the product, which is then in a condition that can be termed as partly degraded.
In an attempt to overcome this drawback and to prevent the burst skins, teguments or other shells from polluting the application cavity, it bas sometimes been proposed to place over the products a bell-type cover, made from a microwave transparent material, such as for example glass or plastic material.
When such a cover is used, it is found that the basic phenomenon described hereinabove, and due to the application of the microwaves, still exists, but that the water in vapor phase released by the products tends to liquefy in contact with the cover and to turn into a liquid phase in the confining enclosure, forming a layer of liquid in the tray, in which the product stands at least partly.
If the use of such a cover prevents projections, it does not however solve the fundamental problem of dispersion and loss of some of the principles of the products, or that of extraction of large quantities of water molecules, two factors which are responsible for changing the properties of the products, such as the organoleptic and nutritional properties of food products.
In addition to the aforesaid drawbacks, it must also be noted that the application of microwaves, particularly when cooking meat, does not permit a controlled centripetal progression of the baking of the product, to achieve, depending on the exposure period, a superficial or intermediate baking, or a baking to the core, thus permitting to meet the cooking requirements conventionally called "rare", "medium rare", and "medium".
The prior art, and in particular French Patent Application No. 82 03 328 (2 501 031) has tried to propose a solution to this problem with a steam cooking apparatus, which is insertable in a microwave oven. According to the teaching of said patent, the apparatus comprises a container previous to microwaves and containing, at least partly, another container reflecting the microwaves and having a perforated base spaced from the bottom of the microwave-permeable container, thereby defining a tank capable of storing water. The apparatus is completed by a cover.
Assuming that positive results are obtained with this apparatus, it should be noted that its structure is complex and bulky and presents two negative and redhibitory factors for wide commercial distribution. The first factor is that of cost and practical use, such as for example installation and cleaning. The second factor is linked to the substantial reduction of the actual serviceable cavity and of the application cavities of microwave ovens.
Another prior art solution has been proposed by U.S. Pat. No. 3 854 023 and consists in a container with a cover intended to contain food products. The container and cover are produced in a microwave-permeable and porous material. This technique is not really satisfactory in that it implies the use of an equipment which is not adapted to microwave ovens and in particular which does not allow any variable perception of the heating or baking process cycle.